There are many applications employing antennas for transmitting and receiving electromagnetic signals in which the defining of antenna gain patterns with maximas for directional transmitting and receiving the signals is a desirable feature. One type of such antenna systems is the active transmit phased array having a plurality of individual antenna elements which are interconnected in ways designed to enable, for example, electronic steering of the radiated beams of electromagnetic energy in space, without physical movement of the whole array. The antenna elements can be distributed uniformly or non-uniformly over a prescribed surface area, and chosen to provide the desired antenna radiation characteristics. The surface may be planar or curved, in more than one plane, and the area's perimeter may be of any shape, e.g., circular, rectangular, or simply a straight line.
The antenna array can be used, for example, in a radar system for estimating the direction-of-arrival of a target. One way to obtain an antenna system with good direction finding ability is to increase angle resolution, for example, by narrowing the main lobe of the radiation pattern of the array. It is known that angle resolution is determined by the array size. For instance, the angular resolution becomes better when the number of the antenna elements is increased, while the distance between the antennas is fixed. However, the increase of the number of the antenna elements can significantly increase the cost of the system. In the limitation of cost, instead of increasing the number of antenna elements, increase of the distance between the antenna elements in the antenna array can also provide increase of the array size. The more separated the antenna elements are the more narrow the main lobe becomes, and thus the better direction finding ability of the system.
Another reason to increase distance between the antenna elements can be associated with the physical size of the antenna elements. In particular, if the wavelength of transmitted and/or received electromagnetic waves is in the millimeter to centimeter region, then it is difficult and sometimes impossible to make the distance between the elements smaller than half a wavelength.
However, the separation of the antenna elements, in an attempt to minimize the number of elements in the array, gives rise to grating lobes generated in the pattern of the radiated energy from the array in the directions other than the desired one. The grating lobes may appear on each side of the main lobe with decreasing amplitude the further away from the main lobe. The two grating lobes closest to the main lobe have the highest amplitude.
The grating lobes can appear in the range of the visible zone (−90°<θ<+90°, where θ is the directional angle, i.e. the scanning angle from “boresight” towards “endfire”) when the antenna elements are spaced apart at the distance more than half a wavelength. In radar applications, if the grating lobes are left in the visible zone as they are, it is not possible to distinguish between targets detected in the main beam and in the grating lobe beams, which results in ambiguities. A target detected in a grating lobe beam will be processed as if it had been received in the main beam, and will be assigned a completely erroneous spatial direction by the radar signal processor. Moreover, grating lobes carry some of the energy to unwanted spatial regions, and thus reduce the operating efficiency of the system.
It is thus desirable to eliminate the grating lobes from the visible zone or to adequately suppress the relative power of the grating lobes with respect to the main beam. For example, if the beam is electronically scanned from the normal towards the tangent to the array surface, in order to avoid the grating lobes in the scanning zone the maximum scan angle can be reduced from ninety degrees to a certain smaller value as the spacing between the antenna elements is greater than one half-wavelength. Thus, there is a trade-off between the maximum scan angle and the minimal distance between the antenna elements.
Various techniques are known in the art for suppressing relative power of grating lobes in an electronically scanned antenna array. One such type of scanned reflector antenna is disclosed in U.S. Pat. No. 3,877,031 to R. Mailloux et al. Grating lobe suppression is realized by adding odd mode power to the fundamental even mode power that normally drives each radiating element of the array. The odd mode power is maintained ±90 degrees out of phase with the even mode power at each radiating element aperture. The ratio of even mode power to odd mode power is varied as a function of main beam displacement from broadside to control the amount of grating lobe radiation.
Another method of grating lobe reduction is disclosed in U.S. Pat. No. 4,021,812 to A. Schell et al, which relates to suppression of side lobes and grating lobes in directional beam forming antennas by the use of a spatial filter. The filter consists of flat layers of high dielectric-constant material separated by air or other low dielectric-constant materials. The filter is placed directly over the antenna radiating aperture, and its dielectric materials have dielectric constant and thickness values that effect full transmission of beam power in a selected beam direction and substantial rejection of it in other directions so as to suppress side and grating lobes.
U.S. Pat. No. 6,067,048 to Yamada describes a radar apparatus comprising a transmitting antenna and a receiving antenna. The receiving antenna is an array having a plurality of antenna elements, wherein each antenna element includes a plurality of elemental antennas, so as to have a predetermined directional pattern. A synthetic pattern of the directional pattern of each antenna element and a directional pattern of the transmitting antenna has a depressed shape of relative power at an angle where a grating lobe of the receiving antenna appears.
There are applications of phased array antennas in which the scanning zone is not symmetrical with respect to the boresight. For example, for a radar system mounted on an aircraft and designed for steering a radiation beam towards the ground and sweeping the beam through a certain angle, scanning well ahead of the aircraft can sometimes be more important than the scanning behind the aircraft. Likewise, for a radar system mounted on a mast, the scanning in the elevation plane far away of the mast is usually more important than the scanning below the mast.
U.S. Pat. No. 5,006,857 to M. J. DeHart describes a planar microstrip antenna structure for a radar application, which permits the beam to sweep on greater angles from boresight in one direction than in another directions. The planar microstrip antenna structure has individual antenna elements in the form of asymmetrical triangular patches. Each of the antenna elements has a triangular shape with three angles and three sides. One of the angles is approximately 60 degrees. The side opposite the 60-degree angle, referred to as the “base,” is sloped at an angle with respect to the perpendicular of the bisector of the 60-degree angle.
Having the base sloped at a selected angle less than 90 degrees provides an element pattern having a significant beam squint. Further, the element pattern remains within 6 decibels until 70 degrees from boresight. The beam of the array may thus be swept in a selected direction through angles until 70 degrees from boresight.